Referring to FIG. 1, there is shown a somewhat simplified representation of a conventional laser based fiber optic transmission system 50. The transmitter's laser 52 is a diode laser that is housed in a hermetically sealed transistor outline (TO) can 54, herein called the laser housing or the TO-can. The laser housing includes a window 56 through which emitted laser light is transmitted. Typically, a thin film antireflection coating 58 is deposited on the inside surface of the window 56 to reduce reflections from the window 56, and may also include another thin film antireflection coating 60 on the outside surface of the window 56 to reduce reflections from that surface. Each of the thin film antireflection coatings 58, 60 is a conventional optical antireflection film, typically having a thickness of a quarter wave of the transmitted laser light.
The light transmitted by the laser is focused by the lens 62 onto a front end of an optical fiber 64 for transmission through the optical fiber 64 to a receiving device 66. A portion of the light transmitted by the laser 52 is reflected back toward the laser 52 by the near and far end faces of the optical fiber 64, as well as by any other optical discontinuities such as connectors. It is well known that reflections coupled back into a laser cavity cause fluctuations in both the amplitude and phase of the laser output. These fluctuations have several detrimental effects including increased relative intensity noise (RIN), increased spectral width, and increased mode partition noise (for multi-longitudinal mode lasers). In fiber optic communication systems each of these may have an adverse effect on system performance. The most notable manifestation is a reduction in fiber optic link length (i.e., the space between signal repeaters must be reduced to avoid loss of information). In some cases, however, the effects can be so profound as to prevent satisfactory performance altogether over even an arbitrarily short link.
A great deal of energy has been spent attempting to reduce unwanted reflections and their degrading effects. Lasers of all varieties including distributed feedback (DFB) lasers, Fabry-Perot (FP) lasers, and vertical cavity surface emitting lasers (VCSELs) are subject to the negative effects of reflections coupled back into the laser cavity. DFB and FP lasers used in single (spatial) mode links of up to 120 km and more are particularly susceptible. However, the present invention can be used with other types of lasers as well.
Reflections occur wherever there is a discontinuity in index of refraction (n) within an optical link. Such discontinuities generally exist wherever a fiber ends (fiber-air interface), and wherever a connector or splice introduces a slight change in index. In the case of a connectorized link (that is, a link with a removable fiber connector on either end as opposed to one where every connection is spliced) two primary reflections are from the near and far ends of the fiber. The near end reflection results from the air/fiber discontinuity where the laser light is focused onto the fiber, and the far end reflection results from the fiber/air discontinuity where the opposite end of the fiber is plugged into a receiver.
In many cases, the near end reflection dominates all others in terms of its negative effects. In such cases, schemes to reduce the near end reflections can provide sufficient optical isolation. Referring to FIG. 2, traditionally, optical isolation in a fiber optic transmission system 80 is obtained using a 45 degree Faraday rotator 82 sandwiched between two linear polarizers 84, 86 oriented at 45 degrees with respect to each other. The 45 degree Faraday rotator 82 is a magneto-optical device that uses magnetic fields to rotate the polarization state of light. The combination of the two polarizers and the Faraday rotator 82 is called an isolator.
Light propagating from the laser passes through the first polarizer 84. Its polarization state is then rotated 45 degrees by the Faraday rotator 82 so that it is parallel to the second polarizer 86. Reflected light traversing the isolator in the opposite direction, however, is first polarized by the polarizer 86 farthest from the laser. It is then rotated, by the Faraday rotator 82, 45 degrees in the same direction as if it were traversing forward through the Faraday rotator 82. The resulting polarization state is thus perpendicular to the orientation of the polarizer 84 closest to the laser 52 and therefore the reflected light is blocked. Such isolators 82, 84, 86 block all reflected light. They are essentially optical one way valves. They are extremely effective, but are also very expensive. In particular, the inclusion of the Faraday rotator 82 greatly increases the cost of the transmitter.
Another method to reduce near end reflections is shown in FIG. 3. FIG. 3 shows more of the details of a typical laser transmitter housing. This same style of housing may be used in all of the prior art transmitters as well as in the transmitters of the present systems. Since the additional details of the transmitter systems shown in FIG. 3 are not relevant to the present invention, they are not repeated in the other figures.
In the system shown in FIG. 3, the transmitter 90 contains a short length of optical fiber 92, herein called a fiber stub, that has an angled endface 94 at the end closer to the laser 52. The laser and lens are arranged such that the light from the laser is coupled efficiently into the angled end 94 of the fiber stub 92. The angled endface 94 eliminated reflection from that surface by directing the reflection away from the laser. At the other end 96 of the fiber stub 92, reflections are reduced by relying on physical contact between fiber stub and the external fiber 64 of the transmission system. The system shown in FIG. 3 has a few notable weaknesses. First and foremost, the system relies on perfect physical contact between the fiber stub 92 and the user's optical fiber 64. Even a microscopic particle on the surface of the fiber stub 92, which is usually inconvenient to clean, will eliminate physical contact and result in reflections as sever as in the uncorrected system shown in FIG. 1. Secondly, the cost of the fiber stub 92 is often significant. Finally, in some situations, the extra length imposed by the fiber stub is a significant disadvantage.